You know how when a show or movie goes to shit, there's always that one specific line that completely summarizes everything wrong with the writing. It's not necessarily the exact second the show ruined itself, but the line itself just perfectly embodies the bad writing/logic and the fandom uses it as a meme for the rest of time. but not in a loving kind of way like the stupid lines in the star war prequels which are all memable but they don’t ruin the whole plot.  
for eg:  
(I maybe misquoting the lines, I have a bad memory)

1. Star Wars: The Rise of Skywalker - "Somehow, Palpatine returned." I’m still hurt about this so i’m not going to explain.   
2. The boys (season 5) - “ It’s what Clara would have wanted “ . Season 5 is hot mess full of vulgar dialogues that are just there for the sake of vulgarity and sometimes really seems out of character. But in all these this one line is I feel shows how bad the writing is this season.  
3. Batman v Superman - "Save Martha!" I just don’t get the what’s batman’s thinking is this movie. He’s a genius but doesn’t want to investigate a little bit before going full psycho killer on superman. I mean he’s supposed to be a world’s greatest detective couldn’t he first might have talked with superman first?  
4. Game of Thrones (season 8) -This not exactly the quote in the movie but in the post episode behind the scenes videos, the writer David Benioff says "Danny kind of forgot about the Iron Fleet." I mean what the fuck are you talking about David, you wrote this shit. You forgot about it David, don’t put this on Danny. You can also make a case about “ Who has better story than Bran the broken” . Really? Even Sansa had better story than bran.

What are some other examples? 

[Hated Trope] Lines so stupid that perfectly condenses everything wrong with the show/movie and gets memed to death by the fandom